Method of treatment of metallic surfaces

ABSTRACT

A METHOD OF TREATMENT OF METALLIC SURFACES IN ORDER TO IMPROVE THEIR RESISTANCE TO WEAR AND TO SEIZURE CONSISTS OF ELECTROLYZING A PART OF HAVING A METALLIC SURFACE WHICH SERVES AS AN ANODE IN A BATH CONSISTING OF WATER AND ONE OF AMIXTURE OF SALTS CONTAINING AT LEAST ONE ELEMENT BELONGING TO THE GROUP OF ELEMENTS CONSTITUTED BY SULPHUR, SLENIUM, TELLURIUM PHOSPHOROUS, BORON SILICON, FLOURINE, CHLORINE, IODINE, BROMINE, ARSENIC, THE TEMPERATURE OF THE BATH BEING COMPRISED BETWEEN THE AMBIENT TEMPERATURE AND THE BOILING TEMPERATURE OF THE MIXTURE OF SALT OR SALTS AND THE WATER. THE BATH MAY BE COMPOSED OF A MIXTURE OF WATER WITH ALKALINE SULPHIDES, THIOCYANATES OR IODIDES.

United States Patent 3,753,875 METHOD OF TREATMENT OF METALLIC SURFACES Philippe Yves Christian Douet, Saint-Etienne, Loire, France, assignor to Centre Stephanois de Recherches Mecaniques Hydromechanique et Frottement, Andrezieux-Boutheon, Loire, France No Drawing. Filed June 9, 1971, Ser. No. 151,478 Claims priority, application France, June 26, 1970, 7023726; Feb. 4, 1971, 7103716 Int. Cl. C23b 11/00 US. Cl. 20456 R 4 Claims ABSTRACT OF THE DISCLOSURE A method of treatment of metallic surfaces in order to improve their resistance to wear and to seizure consists of electrolyzing a part having a metallic surface which serves as an anode in a bath consisting of water and one or a. mixture of salts containing at least one element belonging to the group of elements constituted by sulphur, selenium, tellurium, phosphorus, boron, silicon, fluorine, chlorine, iodine, bromine, arsenic, the temperature of the bath being comprised between the ambient temperature and the boiling temperature of the mixture of salt or salts and water. The bath may be composed of a mixture of water with alkaline sulphides, thiocyanates or iodides.

Methods of treatment of metallic surfaces have already been proposed for the purpose of improving their qualities of friction and resistance to wear, these methods consisting of introducing a non-metal into the metallic surface treated by electrolysis in a bath of molten salts, the part to be treated being anodic, that is to say connected to the positive pole of a current supply source, the bath being at a temperature at least equal to the melting temperature of the salts.

The parts treated by these known methods may have an excellent resistance to wear and to seizure, but there are cases in which the treatment temperature is incompatible with certain mechanical or physical characteristics of the parts to be treated. For example, certain hardened steels are utilized after tempering at a temperature lower than 150 C. If it is desired to sulphurize these steels in a bath having a base of potassium thiocyanate, the temperature of the bath must be chosen to be higher than this value; in this case, the treatment results in a reduction of the hardness.

In another example, the formation of cadmium iodide at the surface of a part of cadmium from a bath of salts with a base of sodium iodide, can be obtained by electrolysis in a bath at a temperature at least equal to the melting point of this salt, namely 665 C. At this temperature the cadmium is in fusion and this results in a degradation of the surface condition of the parts.

The present invention has for its object a method for carrying out at very low temperatures the treatment of metallic surfaces by the introduction of a non-metal into the superficial layers.

The method according to the invention consists of electrolyzing a part having a metallic surface and acting as an anode in a bath composed of water and a salt or a mixture of salts containing at least one element which belongs to the group of elements constituted by sulphur, selenium, tellurium, phosphorus, boron, silicon, fluorine, chlorine, iodine, bromine, arsenic, the temperature of the bath being comprised between the ambient temperature and the boiling point of the mixture of salts plus water.

In order to produce the treatment bath, water is added to the anhydrous salt in a proportion such that the quantity of salt (or mixture of salts) present in the water is 3,753,875 Patented Aug. 21, 1973 at least equal to the limit of solubility of the said salt or mixture of salts.

The mixture thus made is brought up to a temperature at most equal to its boiling temperature, and the initial quantity of water is maintained in the bath by an appropriate means which effects a continuous or discontinuous addition of water.

The part in which the surface to be treated is immersed in the bath thus made is subjected to electrolysis in the bath by being connected to the positive pole of a current supply source, at a current density comprised between 0.1 and 30 amperes per sq. dm., for a period comprised between 1 and 60 minutes.

After treatment, there is observed on the surface of the part a layer with a base of one or a number of compounds between the metal forming the part itself and one or a number of the anion and/or cation elements added by the salt or the mixture of salts, for example iron sulphide and double iron sulphide and sodium sulphide in the case where a steel part is treated in a bath consisting of a mixture of water and sodium sulphide. This layer, which is perfectly adherent to the base metal, gives the treated parts excellent anti-seizure and anti-wear properties.

In an advantageous form of embodiment of the invention, provision is made for subjecting the metallic surface, after the electrolytic treatment, to a treatment carried out in an atmosphere of a gas which is not liable to react either with the deposited layer or with the support at a temperature comprised between that at which the electrolysis in a saturated aqueous solution is effected and 850 C. This heat treatment has a purpose of increasing the surface hardness by dilfusion of the non-metal towards the interior of the part, while forming a hardness gradient which decreases from the surface towards the interior of the said part.

As a result of this heat treatment, there is observed a definite improvement in the resistance to wear and to seizure.

It will be noted that the invention comprises all applications to mechanical parts.

The invention will' be illustrated by the following examples, given without any limitative sense, in which the friction tests relate to the Faville-Levally test in which a cylindrical test piece of 6.5 mm. in diameter rotates while gripped between two jaws in the form of a V at 90.

EXAMPLE 1 The part to be treated is made of nickel.

The bath employed is composed of of a salt or mixture of salts containing at least one of the elements: sulphur, selenium, tellurium, phosphorus, boron, silicon, fluorine, chlorine, iodine, bromine, arsenic, and 20% of water. This salt or mixture of salts may comprise in particular an alkaline sulphide or thiocyanate or iodide.

More particularly, in this example the bath utilized is composed of:

80% of sodium sulphide, 20% of water.

It will be noted that the quantity of salt present in the Water is at least equal to the limit of solubility of this salt in water.

The time of treatment is comprised between 0.1 and 60 minutes and is preferably equal to 15 minutes. The direct current density is comprised between 0.1 and 30 amperes per sq. dm. and is preferably equal to 0.6 A. per sq. dm. The part to be treated plays the part of an anode.

The treatment temperature is comprised between ambient temperature and the boiling temperature of the mixture of salt plus water and is more particularly maintained at 110 C. The slow evaporation of the water initially contained in the bath is compensated by a continuous addition of water at the rate of 30 cu. cm. per hour and per litre of mixture.

After treatment, there can be seen at the surface of the metal part a layer composed of:

70% by weight of nickel sulphide; 30% by weight of the double sulphide of nickel and sodium.

During the Faville test, a test piece of nickel thus treated creeps without seizure after 90 seconds operation for a gripping load of the steel jaws on the test piece of 6,000 N, whereas an untreated test piece seizes instantaneously.

EXAMPLE 2 The part to be treated is of 16 NC 6 steel, case hardened and tempered (having a composition of 011% C, 0.3% Si, 0.8% Mn, 1% Cr, 1.25% Ni, the remainder iron).

The bath employed is composed of:

68% of potassium thiocyanate, 22% of sodium thiocyanate, 10% of water.

The temperature of the bath is maintained at 130 C. The bath is kept homogeneous by gaseous agitation. The metal crucible containing the bath is used as a cathode, and the test sample is placed at the anode. The directcurrent density is 5 a. per sq. dm.

The slow evaporation of the water initially contained in the bath is compensated by an addition of water at a rate of 45 cu. cm. per hour and per litre of mixture.

The Faville friction test gave the following figures: whereas a test sample of 16 NC 6 case-hardened, tempered, ground and not treated seizes-up during the first seconds of the test, the test sample treated under the above conditions creeps without seizure after 75 seconds operation when the gripping load of the jaws on the test sample is 5,500N.

EXAMPLE 3 The part to be treated is in this case a cylindrical test piece of 6.5 mm. in diameter, and 40 mm. in height, in titanium alloy TA6V (Afnor Standard), having the composition: 0.07% C, 0.22% Fe, 6.24% Al, 3.93% V, 0.30% impurities, the remainder being titanium. The electrolytic bath is a solution of potassium iodide dissolved in water to saturation and brought up to a temperature of 115 C.

After this electrolytic treatment, the test piece is heat treated in a non-reactive atmosphere at a temperature comprised between 115 C. and 850 C. and preferably at a temperature of 800 C.

Micrographic examination of this test piece brings into evidence a layer of 340 microns in thickness, the hardness of which passes, as a result of a diffusion of titanium iodide into the alloy, from 1,000 HV at the surface to 457 HV at 250 microns (the hardness in the heart of the test piece being 344 HV, that is to say that of the untreated TA6V).

A test sample of this kind put into rotation on a friction machine of the Faville type, gripped between two jaws, cut with V-notches at 90, of annealed X0 35, was able to rotate for 40 seconds under a load which increased linearly from 2,000 N up to 4,000 N, at which latter load the jaws crept. After the test, the surfaces in friction contact were perfectly polished.

What I claim is:

1. A method of treating metallic surfaces so as to improve their resistance to Wear and to seizure, comprising electrolyzing a part having a surface of a metal selected from the group consisting of iron, nickel and titanium, as an anode in a bath consisting essentially of water and at least one salt whose cation is selected from the group consisting of sodium and potassium and whose anion is selected from the group consisting of sulphide, thiocyanate and iodide, at a temperature between ambient temperature and the boiling temperature of the bath,

the electrolyzing current being direct current of 0.1 to 30 amperes per square decimeter and the duration of said electrolysis being 0.1 to minutes.

2. A method as claimed in claim 1, said water being saturated with said salt.

3. A method as claimed in claim 1, said salt being present in an amount by weight greater than the amount by weight of said water.

4. A method as claimed in claim 1, and thereafter heating said metallic surface to a temperature substantially above the bath temperature but no higher than 850? C.

References Cited UNITED STATES PATENTS 2,949,411 8/ 1960 Beck 2045' 6 R 1,513,120 10/1924 Madsen 204-56 R 737,882 9/1903 Strecher 204-56 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

